In the treatment of waste waters from food processing plants, several methods have been described for removal and reduction of the BOD (biological oxygen demand), oil and grease or total suspended solids. These typically use treatment of the waste water with a coagulant and flocculant at an acidic pH.
In U.S. Pat. No. 5,174,903, Miller describes the use of lignins such as alkali lignins, amine alkali lignins, sulphonated alkali lignins and sulphonic acid, ethoxylated sodium salt of alkali lignin as coagulants and variety A of biological polymers including carrageenan as flocculants. The combination of lignins and biological polymer gives rise to medium to large sized flocs depending on the ratio of lignin to polymer used. Reductions in the BOD and suspended solids are also obtained for the treated clear effluent. The addition of lignin derivatives as a coagulant effectively reduces the amount of biological flocculant required and produces a floc that is apparently more easily settled even though no measure of the settleability was reported.
In a related patent, U.S. Pat. No. 4,966,713, the use of a crude algal composition or an alkali extract of a crude algae composition in effluent treatment has been described. The preferred algae is one belonging to the class of Rhodophyceae or red algae which is the only source of carrageenan. The algal composition is used as both a coagulant and flocculant. Effective reduction of the COD (by 78%) and TSS (by 86%) is claimed for this treatment using a dissolved air flotation (DAF) unit.
Other applications using purified polymers from seaweeds have also been described e.g. FR 2371385 for the use of alginic acid and micro sand.
Due to the emphasis on biologically derived coagulants and flocculants, these prior art methods tend to be expensive. In addition, use of DAF separation appears to be disadvantageous when the effluent has a high fat content, such as in fish processing.
The design of an efficient and inexpensive waste water treatment process is complicated by environmental concerns, for example in the disposal of the components in the effluent. The difficulty is even greater when, as proposed by the inventors of this invention, the solids removed from the waste water are to be used as animal feed.
The difficulties are even greater when large volumes of waste water in the range 2-5 million gallons per day need to be treated. Such volumes require fast settling time. The settling time depends both on the coagulant-flocculant used and on the equipment used in the separation process. While several designs are known from the prior art, the inventors have proposed a novel method to provide rapid batch or continuous processing of large volumes of effluent from food processing plants.